This invention relates to control circuits for power supplies, and more particularly to method and apparatus for limiting surges of current occuring during power-up of the power supply and/or to shutting off the power supply in response to an overvoltage condition in one of the output voltages thereof.
Previously, a series resistor was used to limit the initial current surge occurring when a pair of input lines of a power supply were connected to a battery. Once the input-filter capacitor of the power supply was charged to a prescribed level, an SCR (silicon controlled rectifier) fired for short circuiting the resistor and removing it from the circuit. This results in a fixed voltage drop in the input lines to the power supply. And in telephone systems with racks of equipment in a central office, the ground lead connecting a 48 volt central office battery to the associated power supply may be as much as 8 feet long such that the line resistance is not negligible for surge currents in the hundreds of amperes. The resultant voltage drop in the ground lead may be as much as 20 volts. This means that a 20 volt difference in potential, for example, exists momentarily between equipment panels that are connected to different ends of the ground lead. It has been found that such differences in ground potential are sufficient to cause burn out of integrated circuits in associated equipment. Also, when a printed circuit board containing the power supply is plugged into a connector, and thus to the battery leads, the resultant surge current is sufficient to burn the pins on the board. Overvoltage protection was previously provided by a crowbar circuit which blew the main fuse in the power supply by selectively firing an SCR that was connected across the battery. This can cause a huge current surge, in excess of 200 amperes, which may damage elements of the power supply or associated equipment. Additionally, such a protection circuit cannot be made to automatically reset when the overvoltage condition goes away. Also, removing a circuit board that forms part of the load for the power supply may cause a surge current that is sufficient to trigger the crowbar circuit. In such a circuit arrangement, it is necessary for a repair person to make a trip to the location containing the power supply, or at least to physically locate and replace the fuse. In order to avoid frequent and unwarranted shut downs of equipment including such a crowbar circuit, a wide overvoltage margin must be designed into it. It is desirable to have an overvoltage protection circuit which will disable the power supply for only a short time interval following an overvoltage condition, and which will automatically restart the power supply after a prescribed time interval or when the overvoltage condition is no longer present.